El Niño and Atlantic warming flag Amazon March-May heat up to seven months ahead

The physical link between the Pacific and the Amazon is well understood. During El Niño, unusually warm water in the central and eastern tropical Pacific alters atmospheric circulation and suppresses cloud and rain over parts of the Amazon. More sunlight reaches the surface, and less water is left to cool the land by evaporation ^[10]. El Niño can also warm the tropical Atlantic, and warmth in both oceans can amplify hot, dry conditions over the basin ^[11].

The new part is what the researchers tested that link against. The team, from the ARC Center of Excellence for the Weather of the 21st Century and Brazil's National Institute of Space Research ^[1], looked at extremes, not seasonal averages. "The main mechanisms that affect the Amazon Basin and are related to the Pacific and Atlantic oceans are well established on the seasonal mean timescale," said Andrea Taschetto, an associate professor at UNSW and a contributing author ^[14][8]. "But in this study, machine learning techniques allowed us to examine the relationship with extreme compound events, which is much harder. It is quite surprising that heat extremes are linked to conditions in the Pacific and tropical Atlantic oceans observed up to seven months in advance." ^[9]

That surprise is a reason for care. The study found heat extremes more likely after El Niño warmth and warming in parts of the tropical Atlantic ^[2]. Its clearest result, for March to May, ties heat across much of the basin to ocean conditions up to seven months earlier ^[3]. That is an association pulled out of the historical record. The seasonal physics makes it plausible. It does not show on its own that a given ocean state reliably comes before a given hot spring.

The record is long for climate work and short for extremes. Monthly data from 1950 to 2023 give 74 March-May seasons for each of the six regions ^[4][15]. Extremes are by definition a small fraction of those seasons, so each regional, seasonal link rests on a limited number of events.

The thing this doesn't tell you is how a warning would perform. The press account does not report hit rates, false-alarm rates, or whether the relationships were checked on years held back from the analysis. A land manager needs those figures before acting on a signal seven months out.

There is a second complication. Over recent decades, hot-dry events have come more often and hit harder, and further warming is expected to make them worse. Earlier research also indicates that deforestation can prolong dry spells and raise temperatures ^[13]. A relationship fitted to 1950-2023 is being applied to a basin whose baseline is changing, and the ocean indices do not capture land-use effects.

In my view the March-May heat link is the study's strongest finding. It should go through a formal hindcast test before any warning is built on it. The effort is justified: compound hot-dry events raise fire risk, damage ecosystems, threaten health, food and water supplies, and can weaken the forest's uptake of carbon dioxide ^[12].